1. Technical Field
This invention relates to a method of creating NC data for rough machining and, more particularly, to an NC data creation method for subjecting a workpiece to rough machining using surface data obtained by digitizing a model surface.
2. Background Art
A method is available for creating NC data, which is for performing machining in accordance with the shape of a model, using model-surface data (position indicative of the center of a stylus) digitized by tracer control.
FIGS. 13(a) and (b) are explanatory views in which surface data of a model MDL are digitized by two-way tracing of an X-Z surface. FIG. 13(b) is a partially enlarged view. A stylus STL is transported along the X axis at a predetermined tracing velocity and moves up and down along a model MDL in the Z-axis direction, and three-dimensional position data (coordinates) of the stylus are read in and stored at predetermined times or whenever a see value exceeds a tolerance value. A predetermined amount of pick-feed is performed along the Y axis when the boundary of a tracing region TRR is reached. Thereafter, while similar surface tracing is being performed, the position of the stylus STL is monitored, the surface of the model MDL is digitized by reading in the position data, and NC data are subsequently created using the surface data.
When NC data for rough machining are created using position data digitized by such tracing and machining is performed based on the NC data while effecting a tool-diameter offset using a tool TL whose diameter R is larger than a diameter r of the stylus STL, the tool TL is obstructed by the workpiece WK and may be broken or caused to cut in too deeply (see FIG. 14). Accordingly, the applicant has already proposed a method of checking for interference by a rough machining tool and of correcting tool path in a case where interference will occur.
In accordance with this proposal, and assuming that the model surface has been approximated in step-shaped fashion using the digitized position data and that a predetermined point on each horizontal shape approximating surface is one point constituting a tool path, a point is checked to determine whether the tool interferes with other shape approximating surfaces when the center Pc of the tool is situated at a position obtained by a tool offset from the abovementioned point. In a case where interference will occur, the position Pc of the tool center is moved correctively to a height to avoid interference and the NC data for rough machining are created based on the tool position following the correction.
With this conventional interference checking method, however, a check to determine whether the tool TL will interfere with a shape approximating surface Sa is not carried out while the tool moves from a tool position Pck' (FIG. 15) to a subsequent tool position Pc(k+1)'. Consequently, the tool may interfere with the workpiece at the time of machining, and if machining is performed under these conditions, the tool may break or cut in too deeply.
Accordingly, an object of the present invention is to provide a method of creating data for rough machining in which a tool will not interfere with a workpiece when the tool is moved from one tool position to the next tool position.